is there a way of connecting an external audio interface so that there is a separate audio out for headphones?

without the ability to cue then for real djing such an app could only really be used as a single pitchable player to plug into an external mixer (mixing feature of the app itself could not be used)

mono is NOT an option

I guess whether this is possible depends on the hardware… so are there ANY android phones with standard usb connectors (that I could plug a usb audio interface into)?
… and …are any apps like this able to use such things?

@michaelmd I think the trick here would be to have a bluetooth headset for cueing and you standard output 3.5mm jack to go to your speakers. You can get some ok bluetooth stereo sets. Pity I’m not clever enough to make the app.